Geotechnical Group at Laboratoire Navier

Problems

In the structure of oil and gas wells a cement sheath is placed between the tubular and the rock formation, which plays a major role in the well stability, zonal isolation and casing protection. The loss of cement-sheath integrity can result in the pressurization of annulus, in gas migration up to the surface, and in catastrophic cases, in a blowout and in the total damage of the infrastructure. Analyzing the well safety conditions requires a numerical simulation of the cement sheath response under various thermo-hydro-mechanical loadings. Such a numerical simulation needs an appropriate constitutive law for the cement paste in well conditions. The cement paste is a complex porous material evolving progressively from fluid to solid during hydration. Its behavior and properties at a given time depend on the cement formulation, the hydration conditions (variable along a well) and the history of applied loadings. The material response is largely dependent on the pore pressure and on the couplings between the hydraulic, mechanical, thermal and chemical phenomena. These aspects should be considered in the development of a constitutive law for the cement paste.

Approach

Constitutive modelling of the cement paste is done by combining macro-scale laboratory experiments, microstructural characterization and observations, theoretical and numerical analysis and multi-scale homogenization methods. Thermo-hydro-mechanical loading experiments under various loading paths are performed on both hardened and hydrating cement paste and analyzed in the framework of thermo-poro-elasto-plasticity theory. The time dependent behavior is also explored.

The cement paste microstructure for different hydration temperatures is characterized by a combination of various experimental techniques (e.g. XRD, TGA, 1H NMR, MIP, etc). The hydration kinetics under different temperature and pressures has been explored by performing calorimetry experiments.

Findings

The classical theory of poromechanics is shown to be adequate for describing the cement paste behavior. A complete set of thermo-poro-elastic parameters has been evaluated for a given hardened cement paste. These parameters have then been extrapolated to cement pastes with different w/c ratio and chemical composition using a multiscale micromechanical model. A significant thermal pressurization of the pore fluid is observed during undrained heating which can deteriorate the cement sheath during rapid temperature changes. The thermal expansion coefficient of the cement pore fluid is found to be anomalously higher than the one of the bulk fluid, mainly due to confienement in nanometer size pores. Increasing hydration temperature creates denser C-S-H with lower C/S and H/S ratios, resulting in lower mechanical strength and stiffness. It has been shown that, beyond a given hydration degree thershold, mechanical loading applied on a hydrating cement paste can result in the creation of significant residual strains. A coupled chemo-poro-elasto-plastic model, with a modified Cam-Clay type yield surface, is developed to simulate the macroscopic shrinkage of cement paste.

Impact

The implementation of the proposed model in a numerical simulation tool permits the simulation of the response of the cement sheath in an oil-well from the hydration phase to the hardened state and during the life of the well. The simulation of the hydration process permits to correctly evaluate the initial state of stress in the cement sheath in interaction with tubular and rock formation. The cement sheath behavior submitted to various mechanical, hydraulic and thermal loadings can then be predicted. This will be an important progress for analysis of the oil-well integrity.